On the Mac being remote controlled, go to System Preferences
Sharing tab and select (highlight but do not check) +Screen Sharing+. Click on the +Computer Settings+ button, then check +VNC viewers may control screen with password+ and enter a password. Click OK.
Back on the main window, check the box for +Screen Sharing+ to start this service.
On the Mac (or any computer)
doing the remote controlling, run a "VNC client" program. There are VNC client programs for Mac OS X, Windows, Linux, even Mac OS 9. If the controlling computer is a recent Mac, I recommend +Jolly Fast VNC+. It is actively being worked on by its developer and it is indeed quite fast (and stable). And it's free.
http://www.jinx.de/JollysFastVNC.html